A clutch mechanism of this type is already known and is described in, for example, the specification of German Patent No. 10232017. In this technique, a clutch mechanism is constructed to function when a pressing load is applied to a seat back from a passenger sitting on a vehicle seat. Further, the clutch mechanism is constructed to transmit the pressing load to a lock mechanism capable of maintaining a use condition of a headrest only when the pressing load is applied to the seat back quickly. Thus, when the pressing load is applied to the seat back quickly, the lock mechanism is unlocked by the pressing load. As a result, the headrest can be tilted forwardly.
However, the known clutch mechanism described above is constructed such that the pressing load can be transmit to the lock mechanism via air. Therefore, even when the pressing load is applied to the lock mechanism quickly, the pressing load may not be transmitted to the lock mechanism depending on an ambient temperature.
Thus, there is a need in the art to provide a temperature-independent clutch mechanism which is capable of properly selectively transmitting a pressing load.